Vacuum metallizing of plastic and similar dielectric substrates is disclosed in various forms including U.S. Pat. Nos.:
2,992,125 Fustier
2,993,806 Fisher
3,118,781 Downing
3,914,472 Nakanishi
4,101,698 Dunning
4,131,530 Blum
4,211,822 Kaufman
4,215,170 Oliva
In addition, two reference books are:
Thin Film. Phenomena, Kasturi L. Chopra, Robert E. Kreiger Publishing Company, Huntington, N.Y., 1979. pp. 163-189.
Handbook of Thin Film Technology, Leon I. Maissel and Reinhard Glang, McGraw-Hill Book Company, New York, N.Y., 1970., pp. 8-32 to 8-43.
U.S. Pat. Nos. 4,407,871, 4,431,711 and 4,713,143, assigned to assignee of the present invention and incorporated herein by reference, relate to metallizing of plastic articles and more particularly to the structure and spacing of discrete metal islands used to metallize rather than a continuous metal film. The metallizing is performed utilizing the island coating system as detailed in the aforesaid patents. The system includes generally a primer and a basecoat coating layers, a metallizing layer and a topcoat layer. As disclosed in the above referenced patents, the coating layers contain non-volatile film forming polymers, generally in the range of 10-30%.
The most efficient way to deposit the coating layers of the island coating system is through a spray system. All of the coatings have been applied using a high volume, low pressure spray gun. However, such a system requires the use of organic solvents, generally at 70-90% by weight, as carriers for the coatings in order to be effectively deposited. If the mixture is not properly sprayed the aesthetic properties of the metallized appearance are not achieved. When the materials are sprayed, care must be taken to avoid gravitational flow of the material across the surface of the item being sprayed which can cause coating irregularities such as drips and runs. The material must be even, yet thick enough to cover surface irregularities and yet island formation must occur. Further, using this technology, film builds of 1.5 to 2.0 mils for any coating layer cannot be achieved without significant coating irregularities.
In addition to proper deposition of the coating layers, the appearance and performance of the commercial product, the conductivity of the metal layer, the corrosion resistance of the metal layer and/or the adhesion of the top coat all relate to the structure and spacing of the islands. The above referenced patents provide further teachings related to nucleation and film growth to the desired island structure and spacing that achieves these ends.
In U.S. Pat. No. 5,290,625, assigned to the assignee of the present invention and incorporated herein by reference, the above process is applied to aluminum parts. In a co-pending application, U.S. Ser. No. 248,957, pending, filed the same day as the instant application, assigned to the assignee of the present invention and incorporated herein by reference, the coating layers are modified to include a combined primer/basecoat layer.
U.S. Pat. No. 4,431,711 shows the significant difference in performance to be obtained with a vacuum metallized flexible plastic product, top coated, where the metal particles are coalesced only to the island state instead of being allowed to coalesce as a thin continuous metallic film across which electrical conductivity is established.
The substrate is prepared for metallization by application of primer and basecoat layers in a solvent. The metal is vacuum deposited on the prepared substrate and the separate islands are coalesced from separate nucleation points and are globular or rounded and fused appearing and are part of the nucleation and growth process. The deposited islands are formed, in a preferred embodiment, by indium which is amphoteric and thus has some solubility in both acids and bases. As deposited, the indium metal layer is composed of tiny islands ranging from tiny clusters of 25 angstroms or less in diameter to sizes as large as 4,000 angstroms in diameter. Each of the islands is separated by channels which can be several hundred angstroms wide which produces the desired electrically non-conductive characteristics across the surface of the substrate.
In general, the spaces between the coalesced islands can be filled with the resin of the top coating applied in a solvent, in effect encapsulating the islands and binding them to the substrate surface. The rounded islands are better protected by the resin and the film overall is far more corrosion resistant, surprisingly so. The metal film is much more securely adhered to the substrate--a very significant advantage.
The construction of the metal island structure in U.S. Pat. No. 4,431,711 includes islands that are separated by channels which receive the top coat and allow the resinous film of the top coat to bond to the substrate for the indium island structures. The channels formed between the individual islands also contain many clusters and smaller islands of residual material. This material reduces the total effective area of substrate material to which the top coat can be bonded. Consequently, the resultant vacuum metallized article may be subject to undesirable delamination between the top coat and the substrate material.
The '143 patent adds to the process the step of etching the vacuum deposited material with a solvent which slowly dissolves or removes residual amounts of metal from the channels between the distinct islands. This clears the channels exposing additional bonding surfaces on the substrate for increasing the surface area of adhesion between the substrate and a protective dielectric top coat.
The typical adhesion strength of a top coat material to a base coat material is in the range of two orders of magnitude stronger than the adhesion strength of the top coat to the metal making up the individual island structures separated by the channels. The etch treatment step greatly improves the adhesion of top coat material of the type set forth in U.S. Pat. No. 4,431,711.
While the flexible substrate described in U.S. Pat. No. 4,431,711 has sufficient adhesion to pass most automotive specification tests, it is desirable to improve the adhesion in such articles so that it will consistently pass an X-scribed type taped adhesion test after either Florida exposures or accelerated weathering tests including QUV, weatherometer, xenon, dual carbon arc weatherometer. With increasing emphasis on quality in American made cars, such tests are now part of automotive specifications. By etching the island containing metal layers of the type described in U.S. Pat. No. 4,431,711, an improved adhesion between top coat and base coat materials results so that such X-scribed standards can be met.
Weatherability now includes a requirement for resistance to acid rain. Acid rain is a low pH aqueous solution composed of several acids, primarily nitric and sulfuric acids. Rain drops which remain on the surface of the topcoat have the ability to permeate through the topcoat. As the droplets evaporate, the concentration of acid increases and is therefore more "aggressive". To improve resistance to acid rain, the thickness of the top coat must be increased, thereby reducing permeability. However, as the thickness of the top coat is increased flowout can become poor with its associated "orangepeel" appearance. Other coating irregularities such as drips and runs can occur. Further, "popping" and/or air entrapment increases and gives an appearance that does not provide the aesthetic properties of the metallized appearance.
The current island coating system applies the polymeric constituents of the primer layer, basecoat layer and topcoat layer in organic solvent carriers such as glycol ethers, glycolether acetates, aromatic hydrocarbons and dibasic esters. These solvent carriers pose a waste disposal problem increasing the cost of production significantly. If the organic solvents could be eliminated, while still maintaining the aesthetic properties of the metallized appearance, significant savings as well as ease of waste disposal would be attained.
U.S. Pat. No. 4,923,720 to Lee et al, issued May 8, 1990 and assigned to the Union Carbide Chemicals and Plastics Company, Inc. and incorporated herein by reference, presents a further detailed discussion in columns 1 and 2 on the problems inherent in the use of organic solvent carriers.
Liquid inorganic carriers such as CO.sub.2 can be substituted for organic solvent carriers as disclosed in the Lee et al. '720 patent. In converting gaseous inorganic carriers to the liquid state either pressure or pressure combined with increased temperature can be used to create a "supercritical" fluid or dense gas in which is soluble in the polymer system. The utilization of pressure and increased temperature is expensive not only to produce but to maintain the gaseous inorganic carrier in a liquid state. If pressure alone is used to maintain such a liquid state, there is a further increase in temperature (Ideal Gas Law) that can adversely affect the stability of the polymeric constituents being carried by the liquified inorganic carrier. Additionally, as the pressurized polymeric material is circulated through the spray system, further instability can result.
The Lee et al. '720 patent and a series of related patents as listed below:
______________________________________ U.S. Pat. No. Date of Issue ______________________________________ 5,212,229 May 18, 1993 5,211,342 May 18, 1993 5,203,843 April 20, 1993 5,178,325 Jan. 12, 1993 5,171,613 Dec. 15, 1992 5,141,156 Aug. 25, 1992 5,108,799 Apr. 28, 1992 5,106,650 Apr. 21, 1992 5,066,522 Nov. 19, 1991 5,057,342 Oct. 15, 1991 5,027,742 Jul. 2, 1991 5,009,367 Apr. 23, 1991 ______________________________________
provide information for a system for use of supercritical fluids as diluents in spray coating. The system as taught in the above patents is marketed by Union Carbide Corporation, Danbury, Conn., as UNICARB.RTM.System . Applicant has used the system, and modified the system as taught in the aforementioned patents, to meet the required specifications for parts metallized using the island coating system. There was a variability in appearance of the parts and the UNICARB.RTM. system solvent blend was expensive to use.
It would be useful to be able to use a non-organic or reduced organic solvent system such as the UNICARB.RTM. System to deliver the components of the island coating system producing uniform results at a reduced cost. Further, in utilizing such a system, it is necessary that coatings of 1.5-2.0 mils thickness be deposited without coating defects such as popping, drips, runs and sags.